A few months ago I picked up a Trek 8500 frame that I'm slowly building. Drivetrain and other components have been purchased including Kenda Excavator tires (2.35). I'm confused as to what wheelset to get. There are some wheelsets that would only take "up to" 2.2 or 2.3. Do I have to get a wheelset that is wider than 19mm (internal width) inorder to use these kenda tires?

2.35s, especially in an aggressive pattern like the Excavator, would benefit from a bit wider rim - - like 22mm (the internal width of a standard Rhynolite) or wider. This especially holds true if you have one of the DTC (folding) versions - - the sidewalls are a bit 'squirmy' on those and a narrower rims may tend to accentuate this disturbing tendency.